If water is not flowing a proper regulator will limit the pressure to the set point (On my RV that's 50 PSI) the filter will not have any effect.
When water is flowing the filter MAY restrict flow (Which you see as a drop in pressure) depending on the filter
A cheap regulator will also restrict flow (I use a Watts or Zurin 3/4 inch with the proper hose adapters, About 50-70 dollars at your local plumbing supply)
NOTE: if your filter is restrictive then you can use two filters
One pre-filter set to say 70 PSI (most city water systems are more than that) one post set to 50.. That will improve flow
Recently the park owner where I am, and another RVer were discussing regulators (not needed here) and I mentioned that I normally use one because at least one park I freaquent the line pressure has been measured at something like 112 PSI.
Turns out the other rver had been to a park like that too.. When I ask him were he got kind of close so I named the park.. Same one
Proper place for the regulator is the PARK END of the water hose 110+ will blow many fresh water hoses apart.